The preferred embodiment relates to the control of gear changes in an automatic transmission for a motor vehicle. More particularly, it pertains to a control strategy for preventing noise in a powershift transmission due to a tip-in condition.
A dual clutch powershift transmission includes a first input shaft associated with the odd-numbered forward speed ratios, and a second input shaft 14 associated with the even-numbered forward speed ratios and reverse drive. A dual clutch mechanism produces a drive connection between an engine crankshaft and the first and second input shafts, alternately.
A conventional automatic transmission includes a torque converter, located in the power path between the engine shaft and the transmission input shaft, provides a fluid coupling between the engine and transmission. The torque converter provides a smooth transition between a power-off condition, in which power produced by the engine drives the transmission input shaft, and a power-off condition, in which the drive wheels transmit power to the engine. But a powershift transmission has no torque converter. Instead, the dual input clutch produces a mechanical drive connection between the engine and transmission.
In a powershift transmission tip-in clunk is one of most difficult challenges due to absence of a torque converter. When the driver tips-in, i.e., depresses the accelerator pedal following a coast condition, gear shift harshness and noise, called clunk, are heard and felt in the passenger compartment due to the mechanical linkage, without a fluid coupling, between the engine and powershift transmission input. Tip-in clunk is especially acute in a parking-lot maneuver, in which a vehicle coasting at low speed is then accelerated in order to maneuver into a parking space.
In order to achieve good shift quality and to eliminate tip-in clunk, a powershift transmission should employ a control strategy that is different from that of a conventional automatic transmission. The control system should address the unique operating characteristics of a powershift transmission and include remedial steps to avoid the objectionable harshness yet not interfere with driver expectations and performance requirements of the powershift transmission. There is a need to eliminate shift harshness and noise associated with tip-in clunk in a powershift transmission.